Electroplated lead-free bump deposition

ABSTRACT

A method of forming a metal feature on a workpiece with deposition is provided. The method includes providing an under bump metal layer for solder of an electronic device on the workpiece, depositing a substantially pure tin layer directly to the under bump metal layer, and depositing a tin silver alloy layer onto the substantially pure tin layer.

BACKGROUND

1. Field

The disclosed embodiments relate generally to a method and apparatus for applying metal structures to a workpiece, and more particularly to a method and apparatus for depositing a lead-free solder into micro-scale patterns in the surface of a workpiece coated with a photo-resist patterning film, and more particularly to a method and apparatus for electroplating tin-silver alloy solder bumps.

2. Brief Description of Related Developments

The semiconductor industry has been working towards eliminating lead in electronics, as required under the European Union's Restriction of Hazardous Substances (RoHS) Directive. The industry is moving faster than the regulation to offer “green” consumer's electronics with lead-free packaging. Electrodepositon of lead-free solder such as using through mask patterned deposition, is a technology capable eof providing tight pitch bumping (connection pitch less than approximately 300 microns) or microbumping for advanced electronic packaging. An alloy of tin (Sn) and silver (Ag) is the leading candidate metal for these applications. Substantially pure tin has many desirable properties of a solder metal, for example fatigue resistance, thermal cycling and ductile mechanical properties, however the industry has found that tin whisker growth in substantially pure tin solder makes it an unreliable joining solder for advanced packaging applications. It has been found that a small addition of silver, between approximately 1% and 4% Ag by weight, may significantly reduce the likelihood of Sn whisker formation in the solder joint. Tin-silver alloy (SnAg) solder plating in a conventional manner is more difficult than substantially pure tin electroplating or lead-tin (PbSn) electroplating because of the large difference in electrochemical reduction potential between tin (−0.130 volts SHE) and silver (+0.799 volts SHE). This reduction potential difference causes Ag⁺ ions in the solution to spontaneously react with metallic Sn and or the stannous ion (Sn⁺²) oxidizing the Sn or Sn⁺² to Sn⁺² or Sn⁺⁴ and thereby immersion depositing metallic Ag on the Sn surface. Similarly the Ag⁺ ion in the plating solution can immersion deposition other metals such as nickel or copper. Chemical suppliers have developed organic molecules that are to complex the Ag⁺ ion to bring its reduction potential close to that of Sn⁺² and thereby stabilize the Ag⁺ ion in the plating solution. The organic Ag⁺ ion complex in the plating solution does not eliminate the likelihood of unwanted Ag immersion deposition on the Under Bump Metal (UBM), which is typically Nickel or Copper, when electroplating SnAg lead free solder on such UBM structures. This unwanted immersion deposition may cause void defects at the UBM/SnAg interface, said voids are observable after reflowing the solder, and such voids can cause mechanical and electrical failures of the chip to package joint. There is therefore a need for an alternate method of electroplating SnAg solder to form reliable lead-free bump attachment to the underlying metal to solve the problem facing the electronics industry as it moves toward eliminating all lead from integrated circuit products. Further, the industry also needs to develop economical methods of replacing the lead-tin (PbSn) plated bump structures with a lead-free (SnAg) plated bump structures. Due to the thigh cost of the Ag-complexor and other components in commercial SnAg plating chemistries, the typical cost of SnAg plated bumps is several multiples of the PbSn bumps. Existing methods of electrodepositing SnAg bumps involve expensive control systems in the manufacturing equipment, for example as described in U.S. patent application Ser. No. 11/840,748, which is hereby incorporated by reference in its entirety discloses a commercial plating equipment with a control system to ensure that a constant alloy compositions is provided in the solder metal throughout the deposition. There is therefore a need for a method of SnAg electroplating that minimizes the use of expensive chemistry while providing a reliable interface between the SnAg and the underlying metal.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and other features of the embodiments are explained in the following description, taken in connection with the accompanying drawings. The technology described above may be better understood by referring to the following description taken in conjunction with the accompanying drawings. In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the technology.

FIG. 1 shows a cross sectional view of prior art after the deposition step;

FIG. 2 shows a cross sectional view of prior art after the deposition step;

FIG. 3 shows a cross sectional view of solder bump after thermal treatment;

FIG. 4 shows a top-down section of prior art showing the presence of voids at the UBM to SnAg interface;

FIG. 5 shows a top-down section of the present disclosed embodiments showing absence of voids at the UBM to SnAg interface;

FIG. 6 shows a cross sectional view of the present disclosed embodiments after the second deposition step;

FIG. 7 shows a commercial wafer electro-deposition machine suitable for a manufacturing process using the present disclosed embodiments;

FIG. 8 shows a electro-deposition module; and

FIG. 9 shows a process flow diagram.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Although the present embodiments will be described with reference to the embodiments shown in the drawings, it should be understood that the embodiments can be embodied in many alternate forms of embodiments. In addition, any suitable size, shape or type of elements or materials could be used. The present disclosed embodiments provide a method of providing a reliable interface between an electrodeposited lead-free solder bump and an underlying bump metal (UBM).

Referring now to FIG. 1, there is shown a cross section of single bump at the workpiece surface where the workpiece has been prepared for electrodeposition. An electrical contact element 101 is substantially surrounded by an insulating film 100, these type of features are disposed in a semi-periodic array over the integrated circuit workpiece, for example a 300 millimeter silicon wafer may have 1,000 to 100,000 of such electrical contact, elements distributed across the surface. It is noted that any suitable workpiece or substrate may be provided, for example, gallium arsenide or otherwise. The workpiece is coated with a seed layer 102 and then coated with photoresist 104 which is photo patterned to provide openings into which an under bump metal 106, such as nickel (Ni) or copper (Cu) or a series of Ni and Cu layers, is electrodeposited. Solder metal 120 is electrodeposited onto the under bump metal 106 using the same resist pattern mask layer 104. For example, U.S. Pat. No. 7,012,333 which is hereby incorporated by reference herein in its entirety teaches deposition of a SnAg solder alloy with the alloy being deposited at lower than the SnAg eutectic point which is about 3.5% by weight.

Referring now to FIG. 2, there is shown another prior art method of providing lead-free bump, for example a SnAg or SnAgCu alloy where more noble substantially pure metal layer(s) 131 are deposited on the under bump metal prior to the deposition of a substantially pure tin layer 130. U.S. Pat. No. 6,596,621, which is hereby incorporated by reference in its entirety, teaches forming a lead-free SnAgCu bump by using a under bump metal layer 106 comprised of about 2 micron thick Ni and then coating layer 106 with Ag/Cu 131 in proportions to the substantially pure Sn 130 necessary to form a SnAgCu alloy bump with proportions of about 3.5% Ag and about 0.6% Cu and with the balance Sn.

Referring now to FIG. 3, the potential drawbacks of these prior art approaches will be discussed where FIG. 3 shows a cross section of the solder bump after the thermal reflow process. A thermal reflow process is advantageous to stabilize the solder bump structure prior to subsequent processing. After the electrodeposition step, the photoresist 104 (not shown) is removed and the seed layer 102 is etched away everywhere except where it is protected by the under bump metal 106. Subsequently the wafer is thermally treated in a so-called reflow process step. Briefly described, reflow involves heating the workpiece in a controlled atmosphere so that the tin-oxides are substantially removed before the solder melts, which may occur between about 221° C. and about 232° C. for SnAg alloy; about 221° C. being the SnAg eutectic at composition of about 3.5% Ag and about 232° C. being the substantially pure Sn melting point, when the solder changes phase from solid to liquid the surface tension causes the metal volume to change shape, transforming into a substantially spherical shape 126 as the liquid surface tension minimizes the surface area. Also occurring at the elevated temperature is the formation of a layer of intermetallic compounds (IMCs) 128 which are a mixture of several alloy phases, for example at a Cu/Sn interface the IMCs will be a combination of Cu₅Sn₆ and Cu₃Sn alloy phases. Also occurring at the elevated temperature is the vaporization and outgassing of various organic molecules that may be incorporated into the solder during the deposition process. These elevated temperature processes are halted by cooling down the wafer or substrate, causing the solder to solidify, wherein the solid solder is composed of many sub-micron sized grains which can have different sizes and compositions. For example, U.S. Pat. No. 6,805,974, which is hereby incorporated by reference herein in its entirety, teaches the importance of controlling the alloy composition and the cool-down rate to avoid the unwanted formation of large Ag₃Sn plate shaped grains and instead form a fine grained dispersion of Sn grains and Ag₃Sn small grains.

The importance of providing a repeatable and well controlled intermetallic structure (IMC) between the underbump metal (UBM) and the solder, along with a well controlled grain structure within the solder, may influence both the mechanical and electromigration reliability of the solder bump. In addition, during cooldown the nucleation and growth of the solder grain structure is strongly influenced by the IMCs that were formed. Prohibiting the presence of Ag away at the underbump metal interface during the initial phase of reflow is advantageous as is demonstrated by comparing FIGS. 4 and 5 which show optical microscope images 230, 240 of bumps that have been lapped and polished to the interface region 232, 242 between the underbump metal and the solder, where light and dark colors correspond to the different materials of solder, UBM, and IMC, where the very dark spots are voids. Using a nickel UBM layer and about a 2.5% Ag alloy single step electrodeposition of SnAg shown in FIG. 4 for example, frequent occurrence of interface voids 234, 236, 238 in the region between the UBM and SnAg may occur. By contrast, the disclosed embodiments using a first layer of substantially pure tin and a second layer of tin-silver repeatedly as shown in FIG. 5, no such occurrence of interface voids occur. The substantially, pure Sn layer/bath may be referred to as, for example, a commercially available substantially pure Sn material or bath such as available from Dow Chemical.

Referring now to FIG. 6, there is shown a single bump structure in cross section. Workpiece 250 is prepared with a structure 252 having electrical contact element 101 that is substantially surrounded by an insulating film 100, where these type of features are disposed in a semi-periodic array where the workpiece is coated with a seed layer 102 and then coated with photoresist 104 which is photopatterned to provide openings into which an underbump metal 106, such as nickel (Ni) or copper (Cu) or a series of Ni and Cu layers, is electrodeposited. It is noted that any suitable underbump metal may be provided. A substantially pure tin layer 121 is electrodeposited using an electroplating bath with a metal ion content containing no other metal ion besides tin. It is noted that the workpiece 250 may be rinsed to remove the electroplating bath. A tin-silver layer 122 is then electrodeposited using the same resist pattern mask layer 104 in another plating bath having a metal ion content including tin and silver ions. The thicknesses of the substantially pure Sn layer, T_(Sn), and of the SnAg layer, T_(SnAg), and the % Ag in the SnAg layer, C_(SnAg), are adjusted to provide a final composition % Ag according to the following equation: % Ag=CSnAg×TSnAg/(TSnAg+TSn).

For example, to achieve a final composition % Ag equal to 1.5% Ag the T_(Sn)=T_(SnAg) and C_(SnAg)=3.0%.

It has been considered to apply substantially pure silver (Ag) and substantially pure tin (Sn) to facilitate fabrication of the SnAg alloy, or even to apply Ag, then Cu, then Sn which would then be reflowed to form a SnAgCu alloy, this method could have particular cost advantages since substantially pure Ag and substantially pure Sn plating materials are less expensive then SnAg alloy plating. When using a combination of substantially pure metal layers it is necessary to apply the more noble metals prior to applying the substantially pure tin for two reasons: (1) electrodeposition of Ag onto a Sn surface is difficult to control because of the problem of uncontrolled Ag immersion deposition on Sn, thereby producing an unstable Sn/Ag interface which will cause production control problems between the deposition step and the thermal treatment reflow step; (2) during the thermal reflow process the substantially pure Ag doesn't melt, instead it dissolves into the Sn, and therefore a Ag metal layer would be unstable on the melted tin solder ball, drifting around during the period between Sn melting and Ag fully dissolving into the Sn. However, to apply the Ag directly on top of the UBM material during the reflow process where the intermetallic layer is formed, the presence of Ag between the Sn and the UBM causes the formation of voids in the intermetallic layer, and these voids reduce the reliability of the solder joint. Because the SnAg materials are several times more expensive than Sn materials the present disclosed embodiments provide some of the economic benefit of the substantially pure Ag and substantially pure Sn method, for example reducing the solder deposition cost by approximately 50% or more, without the associated disadvantage of worsening the solder joint reliability.

Referring now to FIG. 7, there is shown a commercial wafer electro-deposition machine suitable for a manufacturing process using the present disclosed embodiments. The disclosed embodiments may be implemented in a commercially available electrodeposition machine such as the Stratus from NEXX Systems in Billerica Mass. System 200 may incorporate features as disclosed in the International Application WO 2005/042804 A2 published under the Patent Cooperation Treaty and having publication date May 12, 2005 which is hereby incorporated by reference herein in its entirety. System 200 is shown in block diagram form as an exemplary system. It is noted that more or less modules may be provided having different configurations and locations. The industrial electrodeposition machine 200 may contain load ports 206 by which substrates previously patterned with photoresist as described above are inserted and withdrawn from the system. Loading station 204 may have a robotic arm which transfers substrates 278 into substrate-holders 270, 272, 274 which are then transferred by transport 280 to modules 210, 212, 214, 216, 260, 262, 264, 266 and processed in succession, The succession may include a copper (Cu) electrodeposition module 216, a nickel (Ni) electrodeposition module 214, a tin (Sn) electrodeposition module 212, a tin-silver (SnAg) electrodeposition module 210. The substrates may then be returned to the loading station 204 which unloads the substrates and passes them through a substrate cleaning module 202 from which they are returned to the load ports 206. Cleaning steps, using de-ionized water for example, may be disposed before and after the electrodeposition steps, for example, cleaning modules 260, 262, 264, 266 may be provided. Alternately, modules 260, 262, 264 and 266 may be rinse or thermal treatment modules as well as clean modules. Controller(s) 220 may be provided within each station or module to sequence the process and/or transport within the station or module. A system controller(s) 222 may be provided within the system 200 to sequence substrates between the stations or process modules and to coordinate system actions, such as, host communication, lot loading and unloading or otherwise those actions that are required to control the system 200. Controller 222 may be programmable to plate the workpiece with substantially pure tin in process module 212 disposed to support a plating bath having a suitable metal ion content (e.g. such as that described above). It is noted that the process module 212 may include either a pure tin anode or an insoluble platinum-titanium (Pt—Ti) anode. Controller 222 may be further programmable to rinse the workpiece in a rinse tank disposed to support rinsing substantially all of the substantially pure tin plating chemistry from the workpiece. Controller 222 may further be programmable to plate the workpiece with tin and silver in process module 210 disposed to support a plating bath with a suitable metal ion content (e.g. such as that described above). It is noted that the process module may include, for example, an insoluble Pt—Ti anode or any other suitable anode. Controller 222 or any other suitable controller may further be programmable to thermally treat the workpiece in a thermal treatment module disposed to thermally treat the workpiece to cause the tin and tin-silver layers to intermix and form a substantially, uniform tin-silver alloy feature. Controller 222 may be further programmable to deposit copper on the workpiece with copper electrodeposition module 216. Controller 222 may further be programmable to deposit nickel on the workpiece with nickel electrodeposition module 214. Controller 222 may further be programmable to clean the workpiece with clean module 260. In the embodiment shown, four electrodeposition modules 210, 212, 214, 216 and four cleaning modules 260, 262, 264, 266 are shown. It is noted, however, that more or less modules may be provided. By way of example, only tin (Sn) electrodeposition module(s) and tin-silver (SnAg) electrodeposition module(s) may be provided. As a further example, separate tools having tin (Sn) electrodeposition module(s) and tin-silver (SnAg) electrodeposition module(s) may be provided. As a further example, multiple duplicate electrodeposition modules may be provided to allow multiple workpieces to be processed in parallel to increase the throughput of the system. As such, all such variations, alternatives and modifications of system configurations are embraced.

Referring now to FIG. 8, there is shown a block diagram of an exemplary electrodeposition process module 210. Electrodeposition module 210 may incorporate features as do modules found in Stratus tools from NEXX Systems in Billerica Mass. and may incorporate features as disclosed in the International Application WO 2005/042804 A2 published under the Patent Cooperation Treaty and having publication date May 12, 2005 which is hereby incorporated by reference herein in its entirety. Exemplary electrodeposition module has housing 300 which contains fluid 302 where fluid 302 may flow through housing 300 and where fluid 302 may be a circulated electrolyte. Workpiece holder 272 may be removable from housing 300 by handler 280 and may hold substrates 278. Although two substrates are shown, holder may hold more or less substrate(s). Anodes 310, 312 are provided with shield plates 314, 316 and paddle or fluid agitation assemblies 318 and 320. It is noted that more or less assemblies may be provided. For example, a single anode may be provided. By way of further example, the anode may be part of housing 300 or shield plates 314, 316 and paddle or fluid agitation assemblies 318 and 320 may not be provided.

The illustrated process may be performed, such as will be described further below with apparatus 200 for example. As may be realized, controller(s) 220 may be suitably programmed to effect the process at least in part in an automatic manner.

Referring now to FIG. 9, there is shown an exemplary process flow diagram 400 showing a method for forming a lead free solder bump on a workpiece. In accordance with the exemplary embodiment, for example, a workpiece with an electrically conducting seed layer covered by a patterned resist mask layer having a plurality of openings may be provided, block 402, for instance in the apparatus. The workpiece may be immersed, block 404, in a tin plating bath containing, for example, a substantially pure tin anode or an insoluble platinum-titanium anode. In block 404, electrical contact to the seed layer may be formed and electrical potential applied between the workpiece and the anode to cause substantially pure tin to be deposited, for example, between about 2 and about 150 microns of tin to deposit tin the resist pattern features. In block 408, the workpiece may be moved to a rinse tank. In block 410, substantially all of the substantially pure tin plating chemistry from the workpiece may be rinsed. The workpiece may be removed from the rinse tank, block 412, and immersed in a plating bath containing tin and silver ions and an anode (e.g., such as, for example, an insoluble platinum-titanium anode), block 414. Electrical contact to the seed layer may be formed as per block 416, and electrical potential applied between the workpiece and the anode to cause tin-silver alloy to deposit. For example, between about 2 and about 150 microns of a tin-silver alloy may be deposited in the resist pattern features. In block 418, the photoresist patterning layer may be removed, and substantially all of the seed layer not covered by the plated tin and tin-silver alloy may be removed, per block 420. Thermally treating the workpiece such as in block 422, for example, at between about 210° C. to about 230° C. (degrees centigrade), may, cause the tin and tin-silver layers to intermix and form a substantially uniform tin-silver alloy feature as desired. In the exemplary process 400, the tin and tin-silver layers may have any suitable thickness or composition, for example, the tin layer may be about 30 microns and the tin-silver alloy layer is about 30 microns and the tin-silver alloy composition may be between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment. By way of further example, the tin layer may be about 10 microns and the tin-silver alloy layer may be about 10 microns and the tin-silver alloy composition may be between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment. By way of further example, the tin layer may be about one-fourth the thickness of the tin-silver layer. Further, in the embodiments, process 400 may provide more or less steps or one or more steps may be combined in one or more step or process. By way of further example, the tin layer may be about 1 micron or 10 microns and the tin-silver layer may be between about 20 microns to about 120 microns.

In accordance with an embodiment, a method of forming a metal feature on a workpiece with deposition is provided. The workpiece is provided with an under bump metal layer for solder of an electronic device. A substantially pure tin layer is deposited directly to the under bump metal layer. A tin silver alloy layer is deposited onto the substantially pure tin layer.

In the embodiment, substantially all of the substantially pure tin plating chemistry from the workpiece may be rinsed.

In the embodiment, the deposition is accomplished by electrodeposition.

In the embodiment, the under bump metal comprises either copper or nickel.

In the embodiment, an apparatus for forming a lead free solder bump on a workpiece having an electrically conducting seed layer, the electrically conducting seed layer covered by a patterned resist mask layer having a plurality of feature openings is provided. The apparatus has a first plating bath with a metal ion content adapted to deposit a substantially pure tin layer in the resist pattern features. A rinse tank may be provided and adapted to rinse substantially all of the substantially pure tin plating chemistry from the workpiece. A second plating bath is provided with a metal ion content adapted to deposit at tin-silver, alloy layer in the resist pattern features.

In the embodiment, a copper electrodeposition module is provided.

In the embodiment, a copper electrodeposition module and a nickel electrodeposition module are provided.

In the embodiment, a cleaning module is provided.

In the embodiment, an electronic device having a lead free solder feature is prepared by a process having a step of depositing a substantially pure tin layer directly to a layer of under bump metal for solder of the electronic device. A step of depositing a tin silver alloy layer onto the substantially pure tin layer is provided.

In the embodiment, a step of rinsing substantially all of the substantially pure, tin plating chemistry from the electronic device may be provided.

In the embodiment, the deposition is accomplished by electrodeposition.

In the embodiment, the under bump metal comprises either copper or nickel.

In the embodiment, a method for forming a lead free solder bump on a workpiece is provided, the method comprising providing a step of providing the workpiece with an electrically conducting seed layer, the electrically conducting seed layer covered by a patterned resist mask layer having a plurality of feature openings. The workpiece is immersed in a first plating bath with a metal ion content. The method comprises providing electrical contact to the seed layer and providing an electrical potential through the metal ion content of the first plating bath to cause between about 2 and about 150 microns of substantially pure tin to deposit in the resist pattern features. The workpiece is immersed in a second plating bath with a metal ion content. Electrical contact to the seed layer is formed and an electrical potential between through the metal ion content in the second plating bath is provided to cause between about 2 and about 150 microns of a tin-silver alloy to deposit in the resist pattern features is provided.

In the embodiment, the method may include moving the workpiece to a rinse tank, rinsing substantially all of the substantially pure tin plating chemistry from the workpiece is provided, and removing the workpiece from the rinse tank is provided.

In the embodiment, removal of the photoresist patterning layer is provided.

In the embodiment, substantially all of the seed layer not covered by the plated tin and tin-silver alloy is removed.

In the embodiment, thermally treating the workpiece at between about 210 to about 230 degrees centigrade to cause the tin and tin-silver layers to intermix and form a substantially uniform tin-silver alloy feature is provided.

In the embodiment, the tin layer is about 30 microns and the tin-silver alloy layer is about 30 microns, and wherein the tin-silver alloy composition is between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment.

In the embodiment, the tin layer is about 1 micron or about 10 microns and the tin-silver alloy layer is between about 20 microns to about 120 microns.

In the embodiment, the tin layer is 10 microns, and the tin-silver alloy layer is about 10 microns, and wherein the tin-silver alloy composition is between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment.

In the embodiment, the tin layer is about one-fourth the thickness of the tin-silver layer.

In the embodiment, an apparatus for forming a lead free solder bump on a workpiece having an electrically conducting seed layer, the electrically conducting seed layer covered by a patterned resist mask layer having a plurality of feature openings is provided. The apparatus has a controller programmable to plate the workpiece with substantially pure tin in a first process module disposed to support a first plating bath having a metal ion content adapted to deposit a substantially pure tin layer on the workpiece. The controller is further programmable to plate the workpiece with tin and silver in a second process module disposed to support a second plating bath with a metal ion content adapted to deposit a tin and silver layer on the workpiece.

In the embodiment, the controller is further programmable to rinse the workpiece in a rinse tank disposed to support rinsing substantially all of the substantially pure tin plating chemistry from the workpiece.

In the embodiment, the controller is further programmable to deposit copper on the workpiece with a copper electrodeposition module.

In the embodiment, the controller is further programmable to deposit nickel on the workpiece with a nickel electrodeposition module.

In the embodiment, the controller is further programmable to clean the workpiece with a clean module.

In the exemplary embodiment, a method for processing one or more workpieces to electrochemically form a pattern of lead-free bumps on a workpiece is provided. In one embodiment the lead-free bump is formed by a substantially two step deposition process, the first step being through mask deposition of substantially pure tin from an electroplating solution containing tin-ions (e.g. a metal ion content), and a second step being through mask deposition of tin-silver alloy from an electroplating solution containing a controlled mixture of tin-ions and silver ions (e.g. a metal ion content), the two steps being controlled to provide target layer 1 and layer 2 thicknesses, T1 and T2, along with the second step being controlled to provide X % alloy composition, such that after a subsequent thermal treatment the two layers intermix and form a substantially uniform alloy of tin-silver (SnAg), said alloy having a concentration intermediate between the deposited X % Ag in the alloy deposition step and the 0% Ag in the substantially pure tin deposition step. The disclosed embodiments prevent the immersion deposition of noble metal ion, such as Ag, and organic complexor on the Under Bump Material (UBM) surface to eliminate the potential forming of voids between the UBM and solder interface. A less noble metal layer, such as substantially pure Tin, is electrodeposited on the UBM before the lead-free solder alloy of Sn and more noble metal, such as Ag and/or Cu is co-deposited with Sn as a SnAg or SnAgCu alloy to form a bump for electronic packaging.

It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims. 

1. A method of forming a metal feature on a workpiece with deposition, the method comprising: providing an under bump metal layer for solder of an electronic device on the workpiece; depositing a substantially pure tin layer directly to the under bump metal layer; and depositing a tin silver alloy layer onto the substantially pure tin layer.
 2. The method of claim 1, wherein substantially all of the substantially pure tin plating chemistry from the workpiece is rinsed.
 3. The method of claim 1, wherein the deposition is accomplished by electrodeposition.
 4. The method of claim 1, wherein the under bump metal comprises either copper or nickel.
 5. The method of claim 1, wherein the workpiece is thermally treated.
 6. An apparatus for forming a substantially lead free solder bump on a workpiece having an electrically conducting seed layer, the electrically conducting seed layer being covered by a patterned resist mask layer having a plurality of feature openings is provided, the apparatus comprising: a first plating bath with a metal ion content configured to deposit a substantially pure tin layer in the resist pattern features; a second plating bath with a metal ion content configured to deposit a tin-silver alloy layer in the resist pattern features.
 7. The apparatus of claim 6, further comprising a rinse tank configured to rinse substantially all of the substantially pure tin plating chemistry from the workpiece; and.
 8. The apparatus of claim 6, further comprising a copper electrodeposition module.
 9. The apparatus of claim 6, further comprising a copper electrodeposition module and a nickel electrodeposition module.
 10. The apparatus of claim 6, further comprising a cleaning module.
 11. A method for forming an electronic device having a lead free solder feature, the method comprising: depositing a substantially pure tin layer directly to a layer of under bump metal for solder of the electronic device; and depositing a tin silver alloy layer onto the pure tin layer.
 12. The method of claim 11, wherein the deposition is accomplished by electrodeposition.
 13. The method of claim 11, wherein the under bump metal comprises either copper or nickel.
 14. The method of claim 11, further comprising rinsing substantially all of the substantially pure tin plating chemistry from the electronic device.
 15. A method for forming a lead free solder bump on a workpiece, the method comprising: providing the workpiece with an electrically conducting seed layer, the electrically conducting seed layer being covered by a patterned resist mask layer having a plurality of feature openings; immersing the workpiece in a first plating bath, the first plating bath having a metal ion content; providing electrical contact to the seed layer and providing an electrical potential through the metal ion content of the first plating bath to cause between about 2 and about 150 microns of substantially pure tin to deposit in the resist pattern features; immersing the workpiece in a second plating bath with a metal ion content; and forming a electrical contact to the seed layer to form an electrical potential through the metal ion content of the second plating bath to cause between about 2 and about 150 microns of a tin-silver alloy to deposit in the resist pattern features.
 16. The method of claim 15, further comprising removing the photoresist patterning layer.
 17. The method of claim 15, wherein substantially all of the seed layer not covered by the plated tin and tin-silver alloy is removed.
 18. The method of claim 15, further comprising thermally treating the workpiece at between about 210 to about 230 degrees centigrade to cause the substantially pure tin and tin-silver layers to intermix and form a substantially uniform tin-silver alloy feature.
 19. The method of claim 15, wherein the substantially pure tin layer is about 30 microns and the tin-silver alloy layer is about 030 microns, and wherein the tin-silver alloy composition is between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment.
 20. The method of claim 15, wherein the substantially pure tin layer is about 10 microns and the tin-silver alloy layer is about 10 microns, and wherein the tin-silver alloy composition is between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment.
 21. The method of claim 15, wherein the substantially pure tin layer is about 1 micron or about 10 microns and the tin-silver alloy layer is between about 20 microns to about 120 microns.
 22. The method of claim 15, wherein the substantially pure tin layer is about one-fourth the thickness of the tin-silver layer.
 23. The method of claim 15, further comprising moving the workpiece to a rinse tank, rinsing substantially all of the substantially pure tin plating chemistry from the workpiece, and removing the workpiece from the rinse tank.
 24. An apparatus for forming a lead free solder bump on a workpiece having an electrically conducting seed layer, the electrically conducting seed layer being covered by a patterned resist mask layer having a plurality of feature openings, the apparatus comprising: a first process module disposed to support a first plating bath having a metal ion content adapted to deposit a substantially pure tin layer on the workpiece; a second process module disposed to support a second plating bath with a metal ion content adapted to deposit a tin and silver layer on the workpiece; and a controller programmable to plate the workpiece with the substantially pure tin layer in the first process module and to plate the workpiece with the tin and silver layer in the second process module.
 25. The apparatus of claim 24, further comprising a rinse tank disposed to support rinsing substantially all of the pure tin plating chemistry from the workpiece, wherein the controller is further programmable to rinse the workpiece in the rinse tank.
 26. The apparatus of claim 24, further comprising a copper electrodeposition module, wherein the controller is further programmable to deposit copper on the workpiece with the copper electrodeposition module.
 27. The apparatus of claim 24, further comprising a nickel electrodeposition module, wherein the controller is further programmable to deposit nickel on the workpiece with the nickel electrodeposition module.
 28. The apparatus of claim 24, further comprising a cleaning module, wherein the controller is further programmable to clean the workpiece with the cleaning module. 